Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

GALIANO AND BLACK GLACIERS                             87
day 5 to 10 year old aider bushes, some of them still green, and the battered stumps of the earlier alder growth, many of them over 25 years old, all mixed together in the moraine that was sliding off of the glacier margin.
On the west side, also, the marginal stream was extending its valley up into the mountains (PL XXXII, B), baring the ice of its load of marginal debris and, in places, developing a steep ice cliff, over a hundred feet high, down which we could not descend, and forcing us up-stream a quarter of a mile to a more gently-sloping portion from which the moraine had not yet been removed. Here we saw a remarkable exhibition of the way in which the ice margin was being stripped of its moraine cover. While eating luncheon by the bank of the marginal stream, just where the bare ice cliff graded into the moraine-covered portion, we watched several small streams issuing from small crevasses and holes in the ice and descending torrentially over and through the moraine talus at the ice base. All of a sudden a landslide of considerable proportions slid down the ice face where one of the streams was issuing from the moraine (below A, PL XXXIII). It was so charged with water that it formed a veritable mud-flow in which stones a ton in weight were floated. The avalanche reached the marginal stream and in a few seconds forced it to one side and over the place where we had just then been seated. Slide after slide followed, and in less than fifteen minutes the marginal stream was pushed over into the fringing alder thicket, fully fifty feet from the place where it formerly flowed. In this brief interval a fan was built whose dimensions were estimated to be 100 by 50 feet, with a depth of 6 feet at its front margin and 25 to 30 feet in the center. An area of ice of several score square yards was bared by these landslides.
On both margins of the glacier near these marginal streams we encountered a band of crevassed ice which we did not see in 1905. It was not fresh breaking, such as is caused by advance, and, moreover, its marginal position, and the absence of crevassing in the glacier center and up the valley precludes this explanation. The phenomenon seems related to the development of the marginal drainage just described. Part of it may be due to slumping through undermining, but moat of it is apparently due to the removal of debris from older crevasses, probably formed during the last cycle of advance. It is conceived that prior to 1905 the ice was broken, as the Atrevida Glacier was in 1906. This breaking was soon healed by ablation and by the sliding of moraine into the crevasses so that by 1905 they were hidden. Presumably there are similar crevasses all over the deeply moraine-covered ice, but most of them are now buried from view, though here and there one is seen; and there are long, narrow, linear valleys and ridges, which probably mark the sites of still others. The extension of the marginal drainage up the glacier side, doubtless a redevelopment of marginal drainage toward the condition that existed before the advance, has sharpened the cliff and exposed the ice, and from this the streams emerge heavily charged with debris. It is thought that the removal of this debris is opening up the marginal crevasses. Further down the glacier, where the d6bris is well stripped off, many crevasses are revealed. Another indication that the explanation proposed is correct is the fact that water is emerging from orifices part way up the ice cliff, as if from the bottoms of crevasses. In more mature ice drainage the water would find its way to the ice bottom through moulins and emerge from the ice base rather from the middle of the marginal slope.
A third notable change in this region in the interval between our visits of 1905 and 1909 is in the coast line. Just south of the glacier front the coast had been completely altered